* France launched Mali military offensive in January
* U.N. peacekeeping force to be made up of African troops
* Peacekeeping force will be U.N.'s third largest (Adds detail on U.N. special envoy for Mali final paragraph)
By Michelle Nichols
UNITED NATIONS, April 25 (Reuters) - The U.N. Security Council unanimously approved on Thursday the creation of a 12,600-strong peacekeeping force in Mali starting July 1, which will be supported by French troops if needed to combat Islamist extremist threats in the West African country.
France, aided by some 2,000 troops from Chad, began a military offensive in January to drive out Islamist fighters, who had hijacked a revolt by Mali's Tuareg rebels and seized two-thirds of Mali.
The U.N. peacekeeping force - to be known as MINUSMA - will assume authority from a U.N.-backed African force deployed there to take over from the French. Most of the African force, known as AFISMA, is likely to become part of the peacekeeping operation, diplomats say.
The U.N. peacekeeping force in Mali will be the body's third largest, behind deployments in Democratic Republic of Congo and Darfur in Sudan, and cost up to $800 million annually, U.N. officials say.
The resolution contains a caveat that the creation of the peacekeeping force be subject to a review by the 15-member council of security in Mali within 60 days of its adoption.
Mali's Foreign Minister Tieman Hubert Coulibaly told the council that the resolution was "an important step in a process to stem the activities of terrorist and rebel groups."
"This mission ... will be concentrated, amongst other things, on stabilizing the main urban centers in the North, restoring the authority of the state ... the protection of civilians, the promotion and protection of human rights as well as humanitarian assistance," said Coulibaly.
Mali's government hopes to hold elections in July, but some diplomats and U.N. officials said that goal may be ambitious.
France has started withdrawing its 4,000-strong force and plans to have just 1,000 by the end of the year. Paris had said Mali's North was in danger of becoming a springboard for extremist attacks on the region and the West.
"France is committed to standing by the Malian people," said French Foreign Minister Laurent Fabius in a statement.
French forces would be able to intervene to support MINUSMA when peacekeepers are "under imminent and serious threat and upon the request of the secretary-general," according to the resolution.
Russia said on Thursday it was alarmed that there was a growing shift towards a "force aspect" within U.N. peacekeeping operations after the council last month created a special combat force within its peacekeeping mission in Congo to carry out "targeted offensive operations" to neutralize armed groups.
"There must be a clear division between peacekeeping and peace enforcement. This is why we believe that the mandate of MINUSMA does not provide for offensive operations," Russia's U.N. Ambassador Vitaly Churkin told the council after the vote.
U.N. peacekeeping chief Herve Ladsous made clear the Mali force was not a peace enforcement or counter-terrorism mission.
"But it is clear at the same time that in an environment which will certainly see asymmetric attacks the stabilization mission will have to defend itself and its mandate depending on circumstances," Ladsous told reporters after the vote.
Peace enforcement missions allow the use of lethal force in serious combat situations, while peacekeeping operations are intended to support and monitor an already existing ceasefire, diplomats and U.N. officials say.
Hundreds of thousands of Malians have been displaced by the fighting and the country's North remains vulnerable to guerrilla-style counter attacks by Islamist extremists.
Mali was once viewed as an example of a working democracy in Africa but its North has been a center of cross-desert trafficking of drugs, stolen goods and Western hostages. Border towns are used as transit hubs for trans-Sahara cocaine and hashish smuggling.
U.N. Secretary-General Ban Ki-moon is due to appoint a special envoy for Mali and some diplomats say the top candidate is Albert Gerard Koenders from the Netherlands who currently heads the U.N. peacekeeping mission in Ivory Coast. (Additional reporting by John Irish in Paris; Editing by Vicki Allen and Cynthia Osterman)